Scientific work over the past decade has demonstrated that the concentration of NO in human breath can be a good indicator of inflammation in the lungs caused by asthma and other respiratory diseases. As a result, there is presently a need for a simple, lightweight, low cost instrument for the measurement of NO in human breath. This invention addresses that need in particular, but is also applicable to the measurement of NO, NO2 NOx, NOy and various gases that react with ozone that are present in air and other gas mixtures such as cylinders containing compressed gases and petrochemical feedstocks for chemical synthesis.
At present, the concentration of NO in a gas sample such as air is most commonly measured by mixing the gas sample with air or oxygen containing ozone gas at low pressures. In a reaction chamber, nitric oxide molecules react with ozone (O3) molecules, to form nitrogen dioxide (NO2) and oxygen (O2) molecules. A small fraction of those reactions also results in the emission of photons having a red or near-infrared wavelength. The concentration of NO in the gas sample is determined by measuring the intensity of that photon emission. This technique, referred to as the “NO+O3 Chemiluminescence” technique is highly sensitive and widely used in the measurement of NO concentrations in ambient air and in inhaled and exhaled human breath. The principal disadvantages of this technique are: 1) a vacuum pump is required, making the instrument large, heavy and highly consumptive of electrical power; 2) a cooled, red-sensitive photomultiplier tube is required, adding to the bulk and weight of the instrument and making it relatively expensive; and 3) the mixing ratio of ozone required for sensitive detection is high, typically a few percent, and requires a high-voltage (several hundred volts) electrical discharge to produce the ozone, thereby increasing the risk of human exposure to this toxic gas and to the danger of electrical shock.
Another technique for measuring concentrations of NO in air samples involves contacting a gas sample with an alkaline luminol solution. As with the ozone-based method described above, this technique produces chemiluminescence. This approach has the advantage of not requiring a vacuum pump and of detecting photons in the visible region where the photomultiplier tube need not be cooled. However, sensitive detection using this technique requires the use of chromium (VI) oxide (CrO3) to oxidize NO to NO2 prior to contact with the luminol solution, and measures must be taken to eliminate large interferences in the measurement from CO2 and water vapor, both of which are present in exhaled breath at high concentrations.
This invention makes use of the same chemical reaction used in the conventional NO+O3 chemiluminescence instrument commonly used in air pollution monitoring and breath analysis. However, the invention differs significantly from that instrument in that the basis of detection is not chemiluminescence (detection of photons emitted by the reaction). Instead, the invention measures the decrease in the concentration of ozone that occurs in the chemical reaction. Advantages of this invention over the conventional NO+O3 chemiluminescence technique are: 1) the concentration of ozone required is much lower, in the low part-per-million range rather than the percent range; 2) the instrument can be operated at any pressure, such as ambient atmospheric pressure, and therefore does not require a vacuum pump; 3) a photomultiplier tube is not required; and 4) the instrument can be based on the extinction of UV light and, therefore, is potentially self-calibrating, which might eliminate the need for compressed cylinders containing standard concentrations of NO.